Substrate for nitride semiconductor growth

ABSTRACT

A substrate for growth of nitride semiconductor capable of obtaining a high-quality nitride semiconductor crystal layer is provided. A substrate for growth of nitride semiconductor for growth of a nitride semiconductor layer on a sapphire substrate ( 1 ) according to one embodiment of the invention is provided with an Al 2 O 3  layer ( 2 ) as separately provided on the sapphire substrate ( 1 ), an AlON layer ( 3 ) which is the first layer, and an AlN layer ( 4 ) which is the second layer. With respect to the first layer and the second layer, the AlON layer ( 3 ) and the AlN layer ( 4 ) are deposited on the Al 2 O 3  layer ( 2 ) in this order.

TECHNICAL FIELD

The present invention relates to a substrate for growth of nitridesemiconductor to be used for growth of a nitride semiconductor on asapphire substrate.

BACKGROUND ART

Since nitride semiconductors have a band gap in the far infrared toultraviolet wavelength region, they are promising as a material of lightemitting or light receiving devices in that region. Also, the nitridesemiconductors have a wide band gap and have a large breakdown field anda high saturation electron velocity. For that reason, the nitridesemiconductors are also very promising as materials of electronicdevices with high-temperature, high output power and high frequencyoperation. Further, since the nitride semiconductors do not containarsenic (As) and phosphorus (P), as compared with GaAs based or InPbased semiconductors which have hitherto been utilized, they have amerit that they are harmless against the environment and are expected asa semiconductor device material in the future.

As a substrate for epitaxial growth of nitride semiconductor having suchexcellent characteristics, any material having a lattice constant and acoefficient of thermal expansion equal to those of the nitridesemiconductors has not been available yet. For that reason, sapphire,SiC, or Si is mainly used as the substrate.

For epitaxial growth of GaN, AlN, InN and their alloyed crystals, asapphire substrate has hitherto been mainly used. However, there arelattice mismatch of 11 to 23% and a difference in the coefficient ofthermal expansion between the sapphire substrate and the nitridesemiconductor. Accordingly, if the nitride semiconductor is growndirectly on the sapphire substrate, the three-dimensional growth occursso that the flatness of the surface in an atomic level becomes worse.For that reason, there was a problem that the nitride semiconductorgrown on the sapphire substrate has a number of crystal defects.

In the case of the epitaxial growth of a nitride semiconductor on thesapphire substrate, it has been reported that crystallinity of GaN wasimproved by a method using a buffer layer. Its technologies will bedescribed below.

The first is a growth method of GaN using a low-temperature AlN bufferlayer (see the following Non-Patent Document 1). This method is asfollows. The sapphire substrate was heated up to the temperature around1000° C. for surface cleaning in metalorganic vapor phase epitaxy systemetc., the temperature was then once dropped. Next, a low-temperature AlNbuffer layer was deposited at around 500° C., and the temperature isagain raised. Then, GaN was grown at around 1000° C. The ALN bufferlayer deposited by this method is amorphous and the islands were formedduring the temperature rising step due to the solid phase growth ofamorphous AlN. As a matter of course, the island shape to be formedvaries depending upon the atmosphere in the growth system (apparatus)orthe temperature rising rate during the temperature rising. At thebeginning of growth of the GaN layer at high temperatures, this islandbecomes a nucleus, whereby the GaN layer undergoes crystal growth.During that crystal growth, flattening of the GaN layer advances due tothe coalescence. GaN undergoes two-dimensional crystal growth on theflattened GaN layer.

The second is a growth method of GaN using a low-temperature GaN bufferlayer (see the following Non-Patent Document 2). This method is asfollows. The sapphire substrate was heated up to the temperature around1000° C., the temperature was then once dropped. Next, a low-temperatureGaN buffer layer was deposited at around 500° C., and the temperature isagain raised. Then, GaN was grown at around 1000° C. Since GaN isdecomposed easily at high temperature as compared with AlN, the nucleusformation in the temperature rising step is not always the same as inthe case of AlN, but the subsequent growth process is substantially thesame.

Incidentally, in the crystal growth of nitride semi-conductors otherthan GaN, the same methods as in those described previously areapplicable, too. For example, in growth of Al_(1−x)Ga_(x)N (0≦x<1) orIn_(1−x)Ga_(x)N (0≦x<1) crystals, a low-temperature GaN buffer layer isdeposited on the sapphire substrate, then GaN, and Al_(1−x)Ga_(x)N orIn_(1−x)Ga_(x)N was grown. In particular, a method of growth ofAl_(1−x)Ga_(x)N crystals is described in the following Non-PatentDocument 3.

As described previously, in all of these growth methods, the bufferlayer was aimed to achieve lattice matching with the GaN layer, butlattice matching with the substrate was not taken into consideration.

Also, even if the buffer layer is deposited at a low temperature, thelow-temperature buffer layer is amorphous and solid phase growth occursat the time of temperature rising. For that reason, the lattice mismatchbetween the buffer layer and the substrate still exists, it is difficultto effectively suppress the generation of crystal defects, and threadingdislocation of 10⁹ to 10¹⁰ cm⁻² exists usually. It is well known thatthis dislocation deteriorates the characteristics of a fabricateddevice. For example, shortening of the life of laser and an increase ofleak current and a lowering of breakdown voltage of the device. Also,diffusion or segregation of impurities may possibly be promoted due tothe existence of the dislocation. Accordingly, reducing the dislocationdensity in the nitride semiconductor layer is very important forimproving the device characteristics, realizing devices which have notbeen attained so far due to influences of the dislocation and enhancingthe controllability in fabrication of a device structure in crystalgrowth.

Accordingly, the invention is aimed to provide a substrate for growth ofa nitride semiconductor capable of obtaining a high quality nitridesemiconductor crystal layer.

Non-Patent Document 1:

H. Amano, N. Sawaki, I. Akasaki, and Y. Toyoda, “Meta illustrated in theforegoing embodiment, the GaN buffer layer 9 (thickness: 1 μm), then⁺-type GaN subcollector layer 10 (thickness: 1 μm), the n⁻-type GaNcollector layer 11 (thickness: 0.5 μm), the p-type GaN base layer 12(thickness: 0.08 μm), the n⁻-type Al_(1−x)Ga_(x)N emitter layer 13 (0≦x<1) (thickness: 0.05 μm), and the n⁺-type GaN contact layer 14(thickness: 0.1 μm) were grown by the metalorganic vapor phase epitaxy.In this case, the growth sequence is a method in which the substrate 6for growth of nitride semiconductor was introduced into a growthfurnace, the temperature was then raised to the growth temperature(1,000° C.) under an ammonia atmosphere, and a source material gas wassupplied. Trimethylgallium, trimethylaluminum and ammonia are used asthe source materials. For dopant of n-type impurities, a Si was used.For dopant of p-type impurities, Mg was used. A mesa structure wasprepared by etching, and ohmic electrodes, i.e., the collector electrode15, the base electrode 16, and the emitter electrode 17, were formed onthe each exposed layers by means of electron beam metal deposition. In acollector current-collector voltage characteristic in common emitterconfiguration of a fabricated transistor, current gain of approximately100 was obtained, and the breakdown voltage was increased toapproximately 200 V with a reduction of the dislocation density asdescribed already being reflected.

DISCLOSURE OF THE INVENTION

One embodiment of the invention is concerned with a substrate for growthof nitride semiconductor to be used for growth of a nitridesemiconductor layer on a sapphire substrate, wherein the substrate isprovided with layers containing N, O and Al as separately provided onthe sapphire substrate. This layer comes into contact with the sapphiresubstrate at the first surface thereof. Also, the foregoing layer isformed such that the proportion of N to the composition ratio of N, Oand Al in the first surface is smaller than that of N to the compositionratio of N, O and Al in the second surface coming into contact with anitride semiconductor layer and that the proportion of O to thecomposition ratio in the first surface is larger than that of O to thecomposition ratio in the second surface.

Another embodiment is concerned with a substrate for growth of nitridesemiconductor to be used for growth of a nitride semiconductor layer ona sapphire substrate, wherein the substrate is provided with an Al₂O₃layer as separately provided on the sapphire substrate and either onelayer of an AlON layer or an AlN layer provided on the Al₂O₃ layer.

A still another embodiment is concerned with a substrate for growth ofnitride semiconductor to be used for growth of a nitride semiconductorlayer on a sapphire substrate, wherein the substrate is provided with anAl₂O₃ layer as separately provided on the sapphire substrate, an AlONlayer which is the first layer, and an AlN layer which is the secondlayer, and has a structure in which the first layer and the second layerare formed on the Al₂O₃ layer in this order.

Here, a cap layer made of Al₂O₃ can be provided as the uppermost layerof the substrate for growth of nitride semiconductor.

According to the embodiments of the invention as described previously,it is possible to provide a substrate for growth of nitridesemiconductor capable of obtaining a high-quality nitride semiconductorcrystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of a substrate for growth ofnitride semiconductor according to one embodiment of the invention;

FIG. 2 is a cross-sectional schematic view of a structure in which a GaNlayer was epitaxially grown on a substrate for growth of nitridesemiconductor according to one embodiment of the invention;

FIG. 3 is a cross-sectional schematic view of a substrate for growth ofnitride semiconductor according to one embodiment of the invention;

FIG. 4 is a cross-sectional schematic view of a substrate for growth ofnitride semiconductor according to one embodiment of the invention;

FIG. 5 is a view to explain the relationship between a time necessaryfor a process of forming a nitride semiconductor on a sapphire substrateand a temperature of a growth furnace;

FIG. 6 is a cross-sectional schematic view of an Al_(1−x)Ga_(x)N(0≦x<1)/GaN heterojunction bipolar transistor structure epitaxiallygrown on a substrate for growth of nitride semiconductor according toone embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the invention will be described below in detail withreference to the drawings. Incidentally, in the drawings as describedhereinafter, a member having the same functions is given the samesymbol, and its explanation is omitted.

EMBODIMENTS

FIG. 1 is a cross-sectional schematic view of a substrate for growth ofnitride semiconductor according to the embodiment of the invention.

In FIG. 1, reference numeral 1 denotes a sapphire substrate (a sapphiresingle crystal substrate); reference numeral 2 denotes an Al₂O₃ layer(an aluminum oxide layer, namely an alumina layer); reference numeral 3denotes an AlON layer (an aluminum oxynitride layer); reference numeral4 denotes an AlN layer (an aluminum nitride layer); reference numeral 5denotes a cap layer (the uppermost layer) made of Al₂O₃; and referencenumeral 6 denotes a substrate for growth of nitride semiconductor.

This embodiment is concerned with a substrate for growth of nitridesemiconductor to be used for growth of a nitride semiconductor layer onthe sapphire substrate 1 and has a structure having the Al₂O₃ layer 2which is separately provided on the sapphire substrate 1, the AlON layer3 which is the first layer, and the AlN layer 4 which is the secondlayer, and the AlON layer 3 and the AlN layer 4 are formed on theforegoing Al₂O₃ layer 4 in this order. Incidentally, even a structure inwhich either one layer of the AlON layer 3 as the first layer or the AlNlayer 4 as the second layer, each having a composition closed to thenitride semiconductor layer, is provided on the Al₂O₃ layer 2, givesrise the effects according to the invention and is effective. Also, thecap layer 5 made of Al₂O₃ is provided as the uppermost layer of thesubstrate 6 for growth of nitride semiconductor.

Incidentally, though the foregoing Al₂O₃ layer 2 is not alwayscrystalline, it is considered that in the layers, a portion in thecrystalline state is lattice-matched with the sapphire substrate 1. Theterms “lattice-matched” as referred to herein means that the latticeconstant is substantially equal.

That is, on the sapphire substrate 1 having a diameter of 2 inches, theAl₂O₃ layer 2 was deposited in a thickness of approximately 5 nm in anAr plasma at room temperature using Al and oxygen as the sources by anECR (Electron Cyclotron Resonance) plasma deposition system. Thereafter,nitrogen was added in the deposition chamber, thereby depositing theAlON layer 3 in a thickness of approximately 10 nm. Next, the AlN layer4 of the same kind as a nitride semiconductor layer (for example, a GaNlayer) to be grown on the substrate 6 for growth of nitridesemiconductor was deposited in a thickness of approximately 10 nm in anAr plasma using Al and nitrogen as the sources. Finally, for the purposeof preventing irregular native oxidation which generates surfaceinstability due to stoichiometric disorder, the cap layer 5 made ofAl₂O₃ was deposited in a thickness of approximately 5 nm in an Ar plasmausing Al and oxygen as the sources. There was thus prepared thesubstrate 6 for growth of nitride semiconductor.

Incidentally, in the foregoing embodiment of the invention, the eachlayers are formed at room temperature (for example, 20° C.), but theinvention is not limited to this temperature. According to oneembodiment of the invention, it is important that the substrate forgrowth of nitride semiconductor according to the invention can be formedeven at room temperature, and with respect to the each layers accordingto one embodiment of the invention, so far as a proper composition ofnitrogen and oxygen can be formed, the temperature may be higher orlower than room temperature (the temperature in the atmosphere).

Also, with respect to the film thickness of the Al₂O₃ layer 2, the AlONlayer 3 and the AlN layer 4, a thickness at which the film can be formedis necessary, and taking into consideration the lattice constant in thec axis of each compound, a single atomic layer is necessary at theminimum. On the other hand, since each layer of the Al₂O₃ layer 2, theAlON layer 3 and the AlN layer 4 is not always crystalline(polycrystalline or amorphous), when the film thickness is too thick, itbecomes difficult to transfer (epitaxially grow) the crystallographicaxis of the sapphire (single crystal) substrate to the nitridesemiconductor. In the invention, taking into consideration thesematters, it is preferable that the thickness of the Al₂O₃ layer 2, theAlON layer 3 and the AlN layer 4 is from approximately 1 nm to 200 nm;and it is more preferable that the thickness of the Al₂O₃ layer 2 isfrom 2 nm to 20 nm, the thickness of the AlON layer 3 is from 2 nm to 70nm, and the thickness of the AlN layer 4 is from 2 nm to 50 nm.

Moreover, in the embodiment of the invention, the film formation of theeach layers is carried out by the ECR plasma deposition system(apparatus), but the invention is not limited thereto. Any apparatus,for example, a MOVPE system having oxygen introduced thereinto, can beemployed so far as the film formation of the each layers according toone embodiment of the invention can be properly carried out.

Next, the experimental results which prove usefulness of the substratefor growth of nitride semiconductor of the invention will be describedbelow.

FIG. 2 is a cross-sectional schematic view of a structure in which a GaNlayer was epitaxially grown on the substrate for growth of nitridesemiconductor of the invention.

In FIG. 2, reference numeral 7 denotes a GaN layer, and referencenumeral 8 denotes an Si-doped n-type GaN layer for measuringconductivity. On the substrate 6 for growth of nitride semiconductor asillustrated in the foregoing embodiment, the GaN layer 7 was grown bythe metalorganic vapor phase epitaxy. In this case, the growth sequenceis a simple method in which the substrate 6 for growth of nitridesemiconductor was introduced into a growth furnace, and thereafter, thetemperature was raised to the growth temperature (1,000° C.) under anammonia atmosphere, thereby growing the GaN layers 7 and 8.Trimethylgallium and ammonia were used as the source materials. Fordoping n-type impurities of the Si-doped n-type GaN layer 8, a silanewas used, thereby forming the Si-doped n-type GaN layer 8.

The grown GaN layers 7 and 8 were subjected to X-ray diffractionmeasurement and Hall effect measurement, thereby evaluatingcrystallinity and electric characteristics. The evaluation ofcrystallinity of the grown GaN layer 7 was carried out by estimating adislocation density in the crystal from a full width at half maximum ofX-ray asymmetric reflection spectrum from (10-10). As a result ofestimating the dislocation density, the dislocation density of therelated art was approximately 2×10⁹ cm⁻². On the other hand, in the caseof the GaN layer 7 prepared using the substrate 6 for growth of nitridesemiconductor of the foregoing embodiment, the discoloration density was9×10⁷ cm⁻² and largely reduced. Also, as a result of the Hall effectmeasurement, the carrier density and mobility of the related art were3×10⁷ cm⁻³ and 340 cm²/Vs, respectively. On the other hand, the carrierdensity and mobility of the GaN layer 7 prepared using the substrate 6for growth of nitride semiconductor of the foregoing embodiment were2×10¹⁷ cm⁻³ and 540 cm²/Vs, respectively, and it became clear that thecharacteristics were largely improved.

In the light of the above, according to the substrate for growth ofnitride semiconductor having the each layers of the invention, since thecomposition of nitrogen and oxygen (ratio of the constitutional atoms)is changed from the each layers which are not always crystalline but area layer for the purpose of achieving lattice matching with the sapphiresingle crystal toward the nitride semiconductor to be grown, the latticespacing or distance between constitutional atoms in each layer changes,and the threading dislocation into the nitride semiconductor crystal islargely reduced as compared with the related art. Accordingly, it ispossible to easily prepare a crystal having a dislocation density in theorder of 10⁷ cm⁻². Namely, in the invention, since the dislocationdensity can be uniformly reduced over the whole of the substratesurface, it is possible to design to improve the crystallinity of thenitride semiconductor layer, and such is very industrially effective.Also, since an improvement of the crystallinity is anticipated with areduction of the dislocation density, various ripple effects includingan improvement of the device characteristics and realization of newdevices are expected.

Incidentally, the substrate for growth of nitride semiconductor of theinvention can also be applied to the growth of crystal of nitridesemiconductors other than GaN, Al_(1−x)Ga_(x)N (0≦x<1), In_(1−x)Ga_(x)N(0≦x<1), AlN, InN, etc. Also, the applicable nitride semiconductor layerdoes not rely upon the presence or absence of doping of impurities, thepolarity of a carrier of the semiconductor doped with impurities, andthe composition ratio of a alloyed crystal.

As the general lattice constants of various constitutional elements,etc. of the substrate 6 for growth of nitride semiconductor in theforegoing embodiment, the following values are known.

-   -   Sapphire and Al₂O₃: a=4.758 angstrom, c=12.991 angstrom    -   AlN: a=3.112 angstrom, c=4.982 angstrom    -   GaN: a=3.189 angstrom, c=5.185 angstrom

That is, with respect to the each layers in the substrate 6 for growthof nitride semiconductor in the foregoing embodiment, since thecomposition of oxygen and nitrogen (ratio of the constitutional atoms)is changed from the sapphire substrate 1 into, for example, an oxide, anoxynitride, and a nitride, the lattice spacing or distance betweenconstitutional atoms in each layer changes.

In the light of the above, the invention is concerned with the substrate6 for growth of nitride semiconductor capable of easy growth of anitride semiconductor having high-quality crystallinity to crystalgrowth. For achieving this matter, the invention is characterized inthat in the buffer layer to be formed on the sapphire substrate 1, thecomposition of the buffer layer is changed from the surface contactingwith the sapphire substrate 1 toward the surface contacting with thenitride semiconductor layer. By changing the composition (ratio of theconstitutional atoms) of the buffer layer, the lattice spacing ordistance between constitutional atoms of the buffer layer changes.

Specifically, the invention is concerned with a layered structure of thesapphire substrate 1, the Al₂O₃ layer 2, the AlON layer 3, and the AlNlayer 4. According to one embodiment of the invention, in the foregoingbuffer layer, the layer contacting with the sapphire substrate 1 is theAl₂O₃ layer 2 which is not always crystalline but usuallylattice-matched with the sapphire substrate in the crystalline state,whereby lattice mismatch between the sapphire substrate and the bufferlayer can be reduced although the Al₂O₃ layer 2 is not alwayscrystalline. Also, in the foregoing buffer layer, by deposition of theAlON layer 3 and the AlN layer 4 which are not always crystalline on theAl₂O₃ layer 2 to form a nitride semiconductor layer on the AlN layer 4,lattice mismatch between the buffer layer and the nitride semiconductorlayer can be reduced although the AlN layer 4 is not always crystalline.Incidentally, with respect to the AlON layer 3 and the AlN layer 4,though it is the best mode that these layers are deposited in thatorder, similar effects can be obtained even by a structure using eitherone of the AlON layer 3 or the AlN layer 4 as illustrated in FIGS. 3 and4.

In the light of the above, with respect to the buffer layer to be formedbetween the sapphire substrate and the nitride semiconductor, what isimportant in one embodiment of the invention is not only to reducelattice mismatch of the surface contacting with the nitridesemiconductor but also to separately provide a buffer layer constructedso as to reduce lattice mismatch of the surface contacting with thesapphire substrate. For achieving this matter, according to oneembodiment of the invention, it is essential that in the buffer layer,the composition of nitrogen and oxygen is changed from the surface ofthe buffer layer contacting with the sapphire substrate toward thesurface contacting with the nitride semiconductor. Namely, it isessential to separately provide a buffer layer on the sapphire substratesuch that with respect to the composition of oxygen and nitrogen to becontained in the buffer layer, the buffer layer has an oxygen-richcomposition in the vicinity of the surface of the buffer layercontacting with the sapphire substrate, that the buffer layer has anitrogen-rich composition in the vicinity of the surface of the bufferlayer contacting with the nitride semiconductor, and that thecomposition of nitrogen increases from the vicinity of the sapphiresubstrate toward the vicinity of the nitride semiconductor substrate.

According to one embodiment of the invention, the buffer layer may bemonocrystalline or polycrystalline or may be an intermediate layer. Inthis description, the term “intermediate layer” as referred to hereinmeans a layer in which single crystals, polycrystals, and amorphouscrystals are mixed.

For that reason, according to one embodiment of the invention, asillustrated in FIG. 1, the buffer layer is constructed such that theAl₂O₃ layer 2, the AlON layer 3, and the AlN layer 4 are successivelydeposited and that nitrogen is increased with a gradient from thesapphire substrate 1 toward the GaN layer 7 (not illustrated in FIG. 1).Also, as illustrated in FIGS. 3 and 4, the buffer layer may beconstructed such that the Al₂O₃ layer and either one of the AlON layer 3or the AlN layer 4 are deposited and that the composition of nitrogen isstepwise increased from the sapphire substrate 1 toward the GaN layer 7(not illustrated in FIGS. 3 and 4).

The Al₂O₃ cap layer according to one embodiment of the invention will bedescribed below in detail.

In the case where the cap layer 5 made of Al₂O₃ is provided as theuppermost layer of the substrate 6 for growth of nitride semiconductor,this cap layer 5 is provided for the purpose of keeping stabilityagainst the external environment, such as one to be exposed to air. Thisstructure is different from the structure using the buffer layeraccording to the related art with respect to the point of the presenceor absence of the cap layer 5. What the cap layer 5 is present enablesone to take out the substrate 6 for growth of nitride semiconductor intothe air atmosphere outside the chamber and makes it easy to handle thesubstrate 6. Incidentally, it is considered that since in this Al₂O₃ caplayer 5 as the final layer, an atmosphere of a nitrogen-containingsubstance such as ammonia is used at the time of crystal growth of thenitride semiconductor, the cap layer 5 is changed into AlN whichconstitutes the AlN layers as its lower layer (however, illustrated asthe Al₂O₃ cap layer 5 in FIGS. 2, 3 and 4 and FIG. 6 as describedlater). Namely, the cap layer 5 plays a dual role of a cap which bringsabout stability and a surface at the time of growth of nitridesemiconductor.

Now, since the related-art growth method of nitride semi-conductorincludes two stages of the growth of the buffer layer as describedpreviously and the temperature-rising and temperature-dropping process,there are encountered such problems that the growth process iscomplicated and that a time necessary for a process of forming a nitridesemiconductor on a sapphire substrate (also referred to as “growth time”in this description) is long.

FIG. 5 is a view to explain the relationship between a time necessaryfor a process of the growth of a nitride semiconductor on a sapphiresubstrate and a temperature of a growth furnace. In FIG. 5, referencenumeral 51 shows a change with time of the temperature of a growthfurnace in the related-art first process in which the temperature israised to a temperature A (1,000° C. to 1,100° C.), thereby cleaning thesapphire substrate, and the temperature is dropped to a temperature B(400° C. to 600° C.), thereby depositing the buffer layer on thesapphire substrate. Reference numeral 52 shows a change with time of thetemperature of a growth furnace in the related-art second process inwhich the temperature is raised to a temperature B (400° C. to 600° C.),thereby depositing the buffer layer on the sapphire substrate. Referencenumeral 53 shows a change with time of the temperature of a growthfurnace according to one embodiment of the invention in the thirdprocess in which the temperature is raised to a temperature C (1,000°C.), thereby growing the nitride semiconductor on the substrate forgrowth of nitride semiconductor.

According to the related art, in growth of the nitride semiconductor onthe sapphire substrate, the first process or second process isperformed, thereby forming the buffer layer on the sapphire substrate,and the third process is performed, thereby growing the nitridesemiconductor on the buffer layer. At this time, as illustrated by thereference numeral 51 or the reference numeral 52, since the inside ofthe growth furnace is set up at high temperatures before growth of thenitride semiconductor, deterioration of the growth apparatus ispromoted. It is considered that the degree of deterioration of thegrowth apparatus is influenced in an exponential function manner by theuse temperature and use time.

On the other hand, according to one embodiment of the invention, sincethe substrate for growth of nitride semiconductor can be prepared atroom temperature, the growth furnace does not experience the temperaturechange as illustrated by the reference numeral 51 or reference numeral52. Thus, as illustrated by the reference numeral 53, thetemperature-rising and temperature-dropping process for forming thebuffer layer, which has hitherto been required, is not required, and byraising the temperature to the growth temperature of the nitridesemiconductor (1,000° C.), it is possible to easily grow the nitridesemiconductor layer. Also, since the time for keeping the temperaturewithin the growth furnace high can be shortened as compared with therelated art, it is possible to realize a long life of the mechanism forheating the substrate within the growth furnace and so on.

In particular, in the case of using the substrate for growth of nitridesemiconductor having an Al₂O₃ cap layer as the uppermost layer, sincethe substrate can be exposed in the air, it becomes possible toseparately form the substrate and the growth of the nitridesemiconductor onto the substrate. Accordingly, in growth of the nitridesemiconductor, by using the substrate for growth of nitridesemiconductor having an Al₂O₃ cap layer as separately prepared, it ispossible to omit the time for forming the buffer layer on the sapphiresubstrate. That is, since the procedures can be started from the thirdprocess, the growth time can be shortened, leading to an improvement ofthe productivity. Accordingly, it becomes possible to simplify thegrowth sequence and to largely shorten the growth time, these aspectshaving hitherto been problems.

Next, an application example of the substrate for growth of nitridesemiconductor of the invention will be described below.

FIG. 6 is a cross-sectional schematic view of an Al_(1−x)Ga_(x)N(0≦x<1)/GaN heterojunction bipolar transistor structure having beensubjected to crystal growth on the substrate for growth of nitridesemiconductor of the invention.

In FIG. 6, reference numeral 9 denotes a GaN buffer layer; referencenumeral 10 denotes an n⁺-type GaN subcollector layer; reference numeral11 denotes an n⁻-type GaN collector layer; reference numeral 12 denotesa p-type GaN base layer; reference numeral 13 denotes an n⁻-typeAl_(1-x)Ga_(x)N emitter layer (0≦x<1); reference numeral 14 denotes ann⁺-type GaN contact layer; reference numeral 15 denotes a collectorelectrode; reference numeral 16 denotes a base electrode; and referencenumeral 17 denotes an emitter electrode.

On the substrate 6 for growth of nitride semiconductor as illustrated inthe foregoing embodiment, the GaN buffer layer 9 (thickness: 1 μm), then⁺-type GaN subcollector layer 10 (thickness: 1 μm), the n⁻-type GaNcollector layer 11 (thickness: 0.5 μm), the p-type GaN base layer 12(thickness: 0.08 μm), the n⁻-type Al_(1−x)Ga_(x)N emitter layer 13(0≦x<1) (thickness: 0.05 μm), and the n⁺-type GaN contact layer 14(thickness: 0.1 μm) were grown by the metalorganic vapor phase epitaxy.In this case, the growth sequence is a method in which the substrate 6for growth of nitride semiconductor was introduced into a growthfurnace, the temperature was then raised to the growth temperature(1,000° C.) under an ammonia atmosphere, and a source material gas wassupplied. Trimethylgallium, trimethylaluminum and ammonia are used asthe source materials. For dopant of n-type impurities, a silane wasused. For dopant of p-type impurities, Mg was used. A mesa structure wasprepared by etching, and ohmic electrodes, i.e., the collector electrode15, the base electrode 16, and the emitter electrode 17, were formed onthe each exposed layers by means of electron beam metal deposition. In acollector current-collector voltage characteristic in common emitterconfiguration of a fabricated transistor, current gain of approximately100 was obtained, and the breakdown voltage was increased toapproximately 200 V with a reduction of the dislocation density asdescribed already being reflected.

1. A substrate system for growth of a nitride semiconductor layer on asapphire substrate, the substrate system comprising: an Al₂O₃ layerprovided on the sapphire substrate; and a second layer including N, Oand Al provided on the Al₂O₃ layer, wherein the second layer contactswith the Al₂O₃ layer at a first surface of the second layer and isformed such that a proportion of N to a composition ratio of N, O and Alin the first surface is smaller than that of N to the composition ratioof N, O and Al in a second surface of the second layer on which anitride semiconductor layer is grown and a proportion of O to thecomposition ratio in the first surface is larger than that of O to thecomposition ratio in the second surface.
 2. A substrate system forgrowth of a nitride semiconductor layer on a sapphire substrate, thesubstrate system comprising: an Al₂O₃ layer provided on the sapphiresubstrate; a second layer including N, O and Al provided on the Al₂O₃layer; and a cap layer made of Al₂O₃ provided on the second layer, thenitride semiconductor being grown on the cap layer, wherein the secondlayer contacts with the Al₂O₃ layer at a first surface of the secondlayer and is formed such that a proportion of N to a composition ratioof N, O and Al in the first surface is smaller than that of N to thecomposition ratio of N, O and Al in a second surface of the second layercontacting with the cap layer and a proportion of O to the compositionratio in the first surface is larger than that of O to the compositionratio in the second surface.
 3. A substrate system for growth of anitride semiconductor layer on a sapphire substrate, the substratesystem comprising: an Al₂O₃ layer provided on the sapphire substrate; asecond layer comprising either one layer of an AlON layer or an AlNlayer, the second layer being provided on said Al₂O₃ layer; and a grownnitride semiconductor layer provided on or above the second layer. 4.The substrate system according to claim 3, further comprising a caplayer made of Al₂O₃ , the cap layer being positioned between the firstlayer and the nitride semiconductor layer such that the nitridesemiconductor layer is grown directly on the cap layer.
 5. A substratesystem for growth of a nitride semiconductor layer on a sapphiresubstrate, the substrate system comprising: an Al₂O₃ layer provided onthe sapphire substrate; an AlON layer provided on the Al₂O₃ layer; anAlN layer provided on the AlON layer; and a grown nitride semiconductorlayer provided on or above the AlN layer.
 6. The substrate systemaccording to claim 5, further comprising a cap layer made of A1 ₂O₃ ,the cap layer being positioned between the AlN layer and the nitridesemiconductor layer such that the nitride semiconductor layer is growndirectly on the cap layer.
 7. A substrate system for growth of a nitridesemiconductor layer on a sapphire substrate, the substrate systemcomprising: a first layer comprised of Al₂O₃, the first layer beingdisposed against the sapphire substrate at a first surface; and a secondlayer including N, O and Al, the second layer being disposed on thefirst layer, the second layer being disposed against a grown nitridesemiconductor layer at a second surface, wherein a proportion of N to acomposition ratio of N, O and Al in the first surface is smaller than aproportion of N to the composition ratio of N, O and Al in the secondsurface and a proportion of O to the composition ratio of N, O and Al inthe first surface is larger than a proportion of O to the compositionratio of N, O and Al in the second surface.
 8. A substrate system forgrowth of a nitride semiconductor layer on a sapphire substrate, thesubstrate system comprising: a first layer comprised of Al₂O₃, the firstlayer being disposed against the sapphire substrate at a first surface;a second layer including N, O and Al, the second layer being disposed onthe first layer; and a cap layer made of Al₂O₃ provided on the secondlayer, the second layer being disposed against the cap layer at a secondsurface and the nitride semiconductor being grown on the cap layer,wherein a proportion of N to a composition ratio of N, O and Al in thefirst surface is smaller than a proportion of N to the composition ratioof N, O and Al in the second surface and a proportion of O to thecomposition ratio of N, O and Al in the first surface is larger than aproportion of O to the composition ratio of N, O and Al in the secondsurface.